Method for producing elemental sulfur from sulfur-containing gases

ABSTRACT

A method for producing elemental sulfur and reduced/oxidized sulfur compounds from sulfur containing gases. The method comprises mixing a primary gas stream of sulfur-containing gases with a secondary gas stream to produce a combined gas stream having a preselected stoichiometry and contacting the combined gas stream with a sorbent/catalyst. The sorbent/catalyst is selected form the group consisting of silica, alumina, and other Claus-type catalysts, sodium/alumina, zinc ferrite, zinc titanate, and other mixed metal oxides, and mixtures thereof.

This application is a continuation of U.S. application Ser. No. 07/729,392, filed Jul. 12, 1991, abandoned, which is in turn a continuation of U.S. application Ser. No. 07/415,680, filed Oct. 2, 1989, abandoned.

TECHNICAL FIELD

This invention relates to a method for converting gaseous sulfur to elemental sulfur and other specific sulfur compounds. In this particular invention sulfur containing gases are mixed with a preselected secondary gas stream and contacted with a catalyst.

BACKGROUND ART

Elemental sulfur is a vital raw material which is used in various industrial processes. For example, sulfuric acid production and vulcanization of rubber require large amounts of elemental sulfur. Most elemental sulfur is recovered from underground deposits using the thermally inefficient Frasch process or recovered as a by-product of natural gas and petroleum processing. However, underground deposits are dwindling and natural gas and petroleum production in the United States is on the decline. This has lead to concern over the future availability of elemental sulfur. In view of this, sulfur containing gaseous emissions from various industrial processes are beginning to be exploited as sources of elemental sulfur.

Processes are available for producing elemental sulfur from the hydrogen sulfide in coal gas, natural gas or oil refinery sour gas, such as the Stretford process. The Stretford process is a low temperature, wet oxidative scrubbing process. However, the Stretford process has numerous mechanical and process chemistry problems which include odor emissions, sulfur plugging in the towers, slurry basin sulfur deposition, and poor hydrogen sulfide removal efficiencies. Other similar processes use aqueous or glycol media which necessarily limits their operating temperature. Certain dry bed processes are commercially available; however, such processes are temperature limited to below the vaporization temperature of sulfur.

The most common process used to convert hydrogen sulfide to elemental sulfur has been that based on Claus technology. Generally, a preconcentration step is required. The process then oxidizes a portion of the feed gas to sulfur dioxide and reacts it with the nonoxidized portion to yield sulfur and water. Catalysts are used to ensure equilibrium at hourly gas space velocities limited to 700 to 1400 scf/cfh. The water generated in the equilibrium reaction limits the reaction. The ratio of hydrogen sulfide to sulfur dioxide must be strictly controlled as well the excess air. Temperatures are limited to about 600° F. to avoid degradation of the catalyst. Reactions are equilibrium controlled so that the Claus process can only operate on concentrated hydrogen sulfide streams and produces a tail gas which requires further treatment. To overcome the equilibrium problem, various tail-end processes must be tacked onto the Claus, for example Claus/Beavon/IFT or Claus/Beavon/Scot or Claus/CBA. Even the newer SuperClaus process requires multi-staging and in any event require hydrogen sulfide as the starting compound. Thus, the Claus technology is complicated, sensitive and expensive.

Still more complex are those regenerable processes such as the Wellman-Lord which converts a very dilute sulfur dioxide gas stream, for example from combustion flue gas, to sulfuric acid or elemental sulfur. The Wellman-Lord process uses sodium sulfite to scrub the flue gas. The bisulfite product is thermally decomposed to yield a concentrated SO₂ gas stream for sulfuric acid production. Partial reduction of the concentrated SO₂ can produce elemental sulfur using augmented Claus technology at the tail end of the process. Another process for the treatment of flue gases is the NOXSO process which uses a reuseable sodium alumina sorbent. The sodium content of the catalyst is near 3 percent and reactivity of the sorbent is limited. In the NOXSO process, the sorption takes place near 244° F. and the bed is then heated to temperatures above 1022° F. for regeneration in a reducing gas. Both the Wellman-Lord type process and the NOXSO process are cyclical in regenerating the sorbent, complex and energy intensive.

Thus, it will be recognized that conventional methods for converting gaseous sulfur to elemental sulfur are typically complex, multistage processes. Also, conventional methods generally are operable at either low temperatures or extremely high temperatures, and often concentration of the sulfur containing gases is required.

Therefore, it is an object of the present invention to provide a method for producing elemental sulfur from sulfur containing gases at high gas space velocities.

It is another object of the present invention to provide a method for producing elemental sulfur from a gas stream containing reduced and/or oxidized sulfur compounds in a direct, continuous process.

A further object of the present invention is to provide a method for producing elemental sulfur from sulfur-containing gases which operates at intermediate temperatures.

Yet another object of the present invention is to provide a method for producing elemental sulfur from sulfur-containing gases which operates efficiently where concentrations of gaseous sulfur in the sulfur-containing feed gas are low.

Another object of the present invention is to provide a method for producing elemental sulfur from sulfur containing gases which can also be used to convert oxidized gaseous sulfur compounds to reduced sulfur compounds and vise versa.

DISCLOSURE OF THE INVENTION

Other objects and advantages will be accomplished by the present invention which provides a method for producing elemental sulfur and specific sulfur compounds from sulfur containing gases. The method generally comprises the mixing of a primary gas stream of sulfur-containing gases with a secondary gas stream to produce a preselected stoichiometry, and contacting the combined gas stream with a catalyst. The catalyst is selected from the group consisting of silica, alumina, other Claus-type catalysts, sodium/alumina, zinc ferrite, zinc titanate and other mixed metal oxides, and mixtures thereof. For the production of elemental sulfur, contacting of the combined gas stream is preferably accomplished at temperatures in the range of about 300° C. to 700° C. depending on the composition of the gas and pressure. Similarly, the contacting of the combined gas stream with the catalyst is preferably accomplished at pressures in excess of approximately 20 atm, but lower pressures can be used at the cost of reduced sulfur yield.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned steps of the present invention will become more clearly understood from the following detailed description of the invention read together with the drawings in which:

FIG. 1 illustrates a schematic diagram of a system for implementing the method of the present invention.

FIG. 2 illustrates a schematic diagram of an alternative system for implementing the method of the present invention.

FIG. 3 is a schematic diagram of a conventional system for producing elemental sulfur from sulfur-containing gases.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention provides a method for converting the sulfur in gas streams containing gaseous sulfur compounds such as hydrogen sulfide, carbonyl sulfide and sulfur dioxide to elemental sulfur. However, as will be discussed below, the method of the present invention can be used, under alternative conditions, to convert oxidized gaseous sulfur compounds such as sulfur dioxide to reduced sulfur compounds such as hydrogen sulfide, and vice versa. Thus, whereas the discussion below is directed primarily to the method as applied to the production of elemental sulfur it will be understood that the present invention is not limited in use to the production of elemental sulfur.

The method of the present invention generally comprises the mixing of a primary gas stream of sulfur-containing gas with a secondary gas stream of selected composition, and contacting the resulting combined gas stream with a preselected catalyst at temperatures of about 300° C. to 700° C. The composition of the secondary gas stream will vary depending upon the composition of the primary gas stream, with the object being that the resulting combined stream have a stoichiometry conducive to substantially complete conversion of the sulfur gases to the desired product, whether it be elemental sulfur or reduced/oxidized sulfur gases. In this regard, if elemental sulfur is the desired product, the mixture of primary and secondary gas streams should have close to such stoichiometry that the sulfur gases can be completely converted to elemental sulfur. Examples of such desirable stoichiometries are as follows:

    SO.sub.2 +2H.sub.2 →2H.sub.2 O+S

    SO.sub.2 +2CO →2CO.sub.2 +S

    SO.sub.2 +2H.sub.2 S→2H.sub.2 O+3S

    H.sub.2 S+(1/2) O.sub.2 →H.sub.2 O+S

Thus, if the primary gas stream contains mainly oxidized sulfur gases the secondary gas stream will be reducing in nature, e.g., containing hydrogen and carbon monoxide, individually or in combination. It is permissible to have oxidizing gases such as steam and carbon dioxide in the secondary stream as long as the secondary gas stream is overall reducing in nature. On the other hand, if the primary gas stream contains mainly reduced sulfur gases, the secondary gas stream will be oxidizing in nature, e.g. containing sulfur dioxide, oxygen, and/or air. Of course, if the primary gas stream already has close to the desired stoichiometry the secondary gas stream is not required.

As noted above, the method of the present invention can also be used to convert oxidized sulfur gases to reduced sulfur gases, and vice versa, this being accomplished by shifting the primary/secondary gas stream mixture stoichiometry toward the desired gas and by selection of the appropriate temperature for the desired conversion. An example of the desired stoichiometries would be:

    SO.sub.2 +3H.sub.2 →H.sub.2 S+2H.sub.2 O

    SO.sub.2 +3CO+H.sub.2 O→H.sub.2 S+3CO.sub.2

The catalyst used pursuant to the method of the present invention is selected from a group consisting of silica, alumina, and other Claus-type catalysis, sodium/alumina, zinc ferrite, zinc titanate and other mixed metal oxides, and mixtures thereof. In the preferred application of the method the catalyst is in the form of pellets and a reactor having a single fixed bed of sorbent is used. It will, however, be recognized by those skilled in the art that reactor modifications including multistage systems, moving-bed and fluidized-bed systems can also be used. The primary gas stream, having been mixed with a small flow of the appropriate secondary gas stream, is contacted with the sorbent/catalyst bed at temperatures of about 300° C. to 700° C. or higher, thereby effecting the conversion of the sulfur gases to elemental sulfur.

The preferred operating conditions under which the combined primary and secondary gas streams are contacted by the catalyst for recovery of elemental sulfur include temperatures on the order of 300° C. to 700° C., and space velocities (in fixed beds) up to and greater than 10,000 scc/(cc.h) , at pressures of about 7 to 20 atm. For example, starting with a primary gas stream containing approximately two percent (2%) SO₂, four percent (4%) CO+H₂, four percent (4%) CO₂ +H₂ O, and the balance N₂, it has been found that it is possible to convert over ninety percent (90%) of the sulfur in the SO₂ to elemental sulfur at 20 atm and 700° C. Particularly preferred parameter combinations include a pressure range of 7-40 atmospheres at a temperature of 449°-700° C. Space velocity can be further controlled to improve yield by providing that the feedstock contacts the catalysts at a space velocity of 1690- 10,000 scc/(cc.h).

However, the preferred operating conditions cited above are simply illustrative of those which maximize the elemental sulfur yield. Conversion to elemental sulfur can be accomplished at temperatures below 300° C., and at pressures below 10 atm. For example, approximately 23% conversion to elemental sulfur from SO₂ has been accomplished at 650° C., at 1.5 atm. Additionally, at times, elemental sulfur conversion has actually increased as temperatures decreased.

The following examples demonstrate various applications of the method of the present invention and typical conversion rates which can be achieved.

EXAMPLE 1

A reducing gas-SO combination containing 5.7 percent H₂ O, 1.8 percent SO₂, 0.6 percent CO₂, 1.5 percent H₂, 2.2 percent CO, and balance N₂ flowing through a fixed bed of a specially prepared and presulfated sodium alumina catalyst, resulted in a 92.3 percent conversion of the inlet sulfur to elemental sulfur at a temperature of 700° C. and a pressure of 20 atm at a space velocity of 1750 scc/(cc.h).

EXAMPLE 2

A reducing gas-SO₂ combination containing 7.7 percent H₂ O, 1.8 percent SO₂, 0.6 percent CO₂, 1.6 percent H₂, 2.1 percent CO, and balance N₂, flowing through a fixed bed of a specially prepared and presulfated sodium alumina catalyst at a space velocity of 1750 scc/(cc.h) , resulted in a 91 percent conversion of the inlet sulfur to elemental sulfur at a temperature of 650° C. and a pressure of 20 atm.

EXAMPLE 3

A reducing gas-SO₂ combination containing 7.7 percent H₂ O, 1.8 percent SO₂, 0.6 percent CO₂, 1.6 percent H₂, 2.1 percent CO, and balance N₂, flowing through a fixed bed of a specially prepared and presulfated sodium alumina catalyst at a space velocity of 1750 scc/(cc.h), resulted in a 23 percent conversion of the inlet sulfur to elemental sulfur at a temperature of 650° C. and essentially atmospheric pressure. Inlet flow was then reduced to give a space velocity of 429 scc/(cc.h) . Conversion to elemental sulfur rose to 46.2%. Raising the bed temperature to a temperature of 700° C. had no significant impact on the extent of conversion. Pressure was then raised to a pressure of 20 atmospheres and flow adjusted to 1753 scc/(cc.h) . Conversion rose to 88.2%.

EXAMPLE 4

An oxidizing gas-H₂ S combination containing, 1.5 percent H₂ S, 0.6 percent COS, 1.8 percent O₂, and balance N₂, flowing through a fixed bed of a specially prepared and presulfated sodium alumina catalyst at a space velocity of 1750 scc/(cc.h) , resulted in a 88.2 percent conversion of the inlet sulfur to elemental sulfur at a temperature of 600° C. and a pressure of 20 atm. Reaction temperature was reduced to a temperature of 540° C. at constant space velocity, and the conversion to elemental sulfur was 89.4 percent. Reaction temperature was reduced to a temperature of 500° C. at constant space velocity, and the conversion to elemental sulfur was 87.6 percent.

EXAMPLE 5

A reducing gas-SO₂ combination containing 11.5 percent H₂ O, 0.87 percent SO₂, 0.25 percent CO₂, 0.68 percent H₂, 0.88 percent CO, and balance N₂, flowing through a fixed bed of a specially prepared and presulfated sodium alumina catalyst at a space velocity of 1830 scc/(cc.h), resulted in a 93.5 percent conversion of the inlet sulfur to elemental sulfur at a temperature of 650° C. and a pressure of 40 atm. Reaction temperature was reduced to a temperature of 602° C. at constant space velocity, and the conversion to elemental sulfur was 93.1 percent. Reaction temperature was reduced to a temperature of 552° C. at constant space velocity, and the conversion to elemental sulfur was 93.4 percent. Reaction temperature was reduced to a temperature of 489° C. at constant space velocity, and the conversion to elemental sulfur was 94.5 percent. Reaction temperature was reduced to a temperature of 449° C. at constant space velocity, and the conversion to elemental sulfur was 91.1 percent. Reaction temperature was reduced to a temperature of 392° C. at constant space velocity, and the conversion to elemental sulfur was 75.9 percent.

EXAMPLE 6

A reducing gas-SO₂ combination containing 24.4 percent H₂ O, 1.5 percent SO₂, 0.5 percent CO₂, 1.3 percent H₂, 1.7 percent CO, and balance N₂, flowing through a fixed bed of a commercial Claus type catalyst, S-100 Alcoa alumina catalyst, resulted in a 93.3 percent conversion of the inlet sulfur to elemental sulfur at a temperature of 650° C. and a pressure of 20 atm at a space velocity of 2150 scc/(cc.h).

EXAMPLE 7

A reducing gas-SO₂ combination containing 7 percent H₂ O, 1.8 percent SO₂, 0.6 percent CO₂, 1.6 percent H₂, 2.0 percent CO, and balance N₂, flowing through a fixed bed of a commercial Claus type catalyst, SP-100 Alcoa sodium alumina catalyst, resulted in a 93.2 percent conversion of the inlet sulfur to elemental sulfur at a temperature of 645° C. and a pressure of 20 atm at a space velocity of 1750 scc/(cc.h). As flow was increased and space velocity ranged to 5060 scc/(cc.h), conversion to elemental sulfur was 90.0 percent.

EXAMPLE 8

A reducing gas-SO₂ combination containing 16 percent H₂, 1.6 percent SO₂, 0.5 percent CO₂, 1.4 percent H₂, 1.8 percent CO, and balance N₂, flowing through a fixed bed of a commercial Claus type catalyst, S-100 Alcoa alumina catalyst, resulted in a 88.8 percent conversion of the inlet sulfur to elemental sulfur at a temperature of 645° C. and a pressure of 20 atm at a space velocity of 5560 scc/(cc.h) . As flow was increased and space velocity ranged to near 10,000 scc/(cc.h) , conversion to elemental sulfur was 85.3 percent. As flow was increased and space velocity ranged to near 20,000 scc/(cc.h) , conversion to elemental sulfur was 52.3 percent.

EXAMPLE 9

A reducing gas-SO₂ combination containing 3.7 percent H₂ O, 1.9 percent SO₂, 0.6 percent CO₂, 1.6 percent H₂, 2.1 percent CO, and balance N₂, flowing through a fixed bed of a commercial catalyst, T-1571 UCI silica, resulted in a 93.2 percent conversion of the inlet sulfur to elemental sulfur at a temperature of 645° C. and a pressure of 20 atm at a space velocity of 1690 scc/(cc.h).

EXAMPLE 10

A reducing gas-SO₂ combination containing 7.9 percent H₂ O, 1.8 percent SO₂, 0.6 percent CO₂, 1.6 percent H₂, 2.0 percent CO, and balance N₂, flowing through a fixed bed of a zinc titanate catalyst, L-3014 UCI, resulted in a 94.42 percent conversion of the inlet sulfur to elemental sulfur at a temperature of 600° C. and a pressure of 20 atm at a space velocity of 1690 scc/(cc.h).

EXAMPLE 11

A gas representative of a gas treated by a first stage of the invented process after sulfur condensation containing 34 percent H₂ O, 0.04 percent SO₂, 0.08 percent H₂ S, 0.4 percent CO₂, and balance N₂, flowing through a fixed bed of a commercial Claus type catalyst, S-100 Alcoa alumina, resulted in a 79.5 percent conversion of the inlet sulfur to elemental sulfur at a temperature of 300° C. and a pressure of 20 atm at a space velocity of 2900 scc/(cc.h). At a reaction temperature of 650° C. at a near constant spaced velocity, the conversion to elemental sulfur was 52.0 percent.

EXAMPLE 12

A potentially reactive gas combination containing 8 percent H₂ O, 1 percent SO₂, 2 percent H₂ S, 37.3 percent CH4, and balance N₂, flowing through a fixed bed of a commercial Claus type catalyst, S-100 Alcoa alumina, resulted in a 98.3 percent conversion of the inlet sulfur to elemental sulfur at a temperature of 400° C. and a pressure of 20 atm at a space velocity of 1890 scc/(cc.h). Under the selected conditions, the methane remained inert.

Referring now to the Figures, in FIG. 1 an example of one preferred application of the method of the present invention is schematically illustrated at 10. For purposes of illustration and by way of one example, the primary gas stream, referenced at 12, is depicted as regeneration off-gas which will be used as one preferred primary gas stream sources, typically provided at a temperature on the order of 700° C., at a pressure of approximately 25 atm, with SO₂ concentrations of approximately 2 mole %. The secondary gas stream, referenced at 14, is depicted as coal gas, the secondary stream 14 being reducing in nature. As illustrated, the primary and secondary gas streams are mixed at 16 and directed into the sulfur production reactor 18 to be contacted by the fixed bed of catalyst therein shown at 19. The resulting H₂ S, SO₂ and S are fed to a sulfur condenser/steam boiler 20 where the molten elemental sulfur condensate is removed through line 21 leaving tail gases of H₂ S and SO₂. The tail gases can then be treated by a conventional Shell Claus Off-gases Treating (SCOT) process 22. As illustrated the H₂ S rich overhead produced from the SCOT process is recycled through line 23 to be mixed with the primary stream 12 such that the further conversion to elemental sulfur can be effected and the clean tail gases, having an environmentally safe sulfur content of less than 250 ppmv, can be released into the environment.

In FIG. 2 an alternative system for applying the method of the present invention is illustrated at 10' wherein 12', 14 and 16' correspond to 12, 14 and 16 of FIG. 1. It will be noted that the system 10' is a multiple bed system including a sulfur production reactor 18' having a catalyst bed 19', an intercooler 20' and a tail-gas polishing reactor 24 having a catalyst bed 25. Thus, it will be recognized that rather than utilizing the SCOT process to clean the tail gas remaining after initial conversion by the reactor 18', the second reactor 24 is used to effect further conversions of sulfur and thereby clean the tail gas being discharged from reactor 24 through line 26 so as to leave only traces of SO₂.

The advantages of the method of the present invention can be readily seen by comparing the systems of FIGS. 1 and 2 with a conventional sulfur producing system based on Claus technology illustrated in FIG. 3. Because a Claus plant is not suited for elemental sulfur production at such low concentrations of SO₂ the SO₂ must be concentrated, as by the Wellman-Lord process, which requires that the sulfur-containing gas first be cooled. Two thirds of the concentrated SO₂ must then be converted to H₂ S before conversion to elemental sulfur can be effected in the three stages of the Claus reactor. By contrast, where the method of the present invention is used, the primary regeneration off-gas need not be concentrated. Regeneration off-gases are typically within the preferred temperature range of the method, and the method is suitable for conversion to elemental sulfur even where SO₂ concentrations are very small. Accordingly, rather than a multiple step, stage by stage process such as those currently used, the method of the present invention is essentially a one step, continuous process which results in elemental sulfur conversion greater than 90%. A second stage is only necessary if over 99% recovery is needed. The method operates over a wide range of temperatures. The process is insensitive to catalyst properties deemed important in the Claus process, such as specific surface area which is adversely affected by thermal gradation.

While one preferred application of the method of the present invention has been shown and described, it will be understood that there is no intent to limit the invention to such disclosure, but rather it is intended to cover all modifications and alternate methods, and alternative gas compositions falling within the spirit and scope of the invention as defined in the appended claims. 

We claim:
 1. A method for producing elemental sulfur and sulfur compounds from sulfur-containing gases comprising:mixing a primary gas stream comprising sulfur-containing gases consisting essentially of sulfur dioxide with a secondary reducing gas stream comprising at least one of H₂ and CO to prepare a gaseous mixture feed material, Contacting a catalyst selected from the group consisting of alumina, with a feedstock consisting essentially of said gaseous mixture feed material at a pressure of 7-40 atmospheres and a temperature of 449°-700° C., and recovering 75.9%-94.5% sulfur of said primary gas stream as elemental sulfur from the gas produced by said catalyst/feed material contacting step.
 2. The process of claim 1, wherein said pressure is in the range of 7 to 20 atmospheres.
 3. The process of claim 1, wherein said feedstock contacts said catalyst at a space velocity of 1690-10000 scc/(cc.h) 